[unreadable] Hepatitis C virus (HCV) can cause cirrhosis and hepatocellular cancer. Understanding the mechanism of HCV replication is of great importance for the development of treatments that will prevent infection or assist those who are already infected. HCV is an RNA virus that expresses a polymerase and a helicase known as NS5B and NS3 respectively. NS5B copies RNA and NS3 unwind structured RNAs. These proteins are essential for HCV replication. NS3 physically interacts with NS5B as well as with NS4A, an HCV protein that has been demonstrated to enhance NS3 RNA-unwinding activity. NS3 affects the polymerization activity of NS5B through an unknown mechanism. Moreover, the binding of NS5B to NS3 may affect the helicase activity of NS3. Therefore, the following aims will be accomplished: (1) Determine how NS3 as well as the NS3/NS4A complex affect the polymerization activity of NS5B. Specifically, we will examine how the helicase activity affects elongation and initiation of polymerization. (2) Determine how NS5B affects the helicase activity of NS3 as well as that of the NS3/NS4A complex. That is, we will determine how the binding of NS5B and its polymerization activity affect the unwinding activity of NS3 and NS3/NS4A. [unreadable] [unreadable] [unreadable]